Personal computers typically connect to an optical disk drive such as a CD-ROM to read data from a compact disk. On the compact disk, data is stored in the form of pits and lands patterned in a radial track. The track is formed in one spiral line extending from the inner radius of the disk to the outer edge. A pit is a location on the disk where data has been recorded by creating a depression in the surface of the disk with respect to the lands. The lands are the areas between the pits in the tangential direction. The reflectivity of the pits is less than the reflectivity of the lands. To store audio or digital information, the length of the pits and lands are controlled according to a predefined encoding format.
When reading information from the disc, light from a laser beam is directed onto the track and the light beam is reflected back to a photo-sensor. Since the pits and lands have different reflectivity, the amount of reflected light changes at the transitions between the pits and the lands. In other words, the encoded pattern of the pits and lands modulates the reflected light beam. The photo-sensor receives the reflected light beam, and outputs a modulated signal, typically referred to as an RF signal, that is proportional to the energy of the light in the reflected light beam.
In FIG. 1, the relationship of the RF signal to the pits and lands is shown. A smaller pit or land decreases both the period and the amplitude of the RF signal. The RF signal in the pits and lands has opposite polarity.
One encoding format used in optical disk systems is eight-to-fourteen modulation (EFM). EFM reduces errors by minimizing the number of zero-to-one and one-to-zero transitions. In other words, small pits are avoided. In EFM, the data signal includes no less than two zeros and no more than ten zeros between logical transitions at the pit edges. A zero is indicated by no change in the energy of the reflected beam for at least two clock periods. A one is indicated by a change in the energy of the reflected light beam, that is, a pit edge. Applying the EFM encoding rules, a pit or land will have a length corresponding to the amount of time for at least two and up to ten clock periods and the electronics will output a corresponding voltage as shown in FIG. 1.
The RF signal needs to be decoded into a serial digital data signal. In one circuit, to decode the analog RF signal, a comparator compares the RF signal to a reference voltage to generate a digital data signal. A bias resistor is coupled to the comparator input receiving the read channel signal to center a DC offset voltage of the read channel signal to be substantially equal to the reference voltage.
Compared to magnetic disk drives, the optical disk is much less sensitive to the effects of contamination such as dust and fingerprints. However, contamination does affect the RF signal because the reflectivity of the disk is changed in the contaminated regions. Although various error correction techniques are applied to the decoded digital data that is derived from the RF signal, these techniques are limited as to the number of errors that can be corrected.
Contamination will change the DC offset voltage of the RF signal. Since the bias resistor is fixed, the bias resistor cannot be adjusted to compensate for the changed offset voltage. Therefore, the digital data signal will have an error that may or may not be corrected.
In view of the foregoing, it would be highly desirable to provide a circuit that adjusts the DC offset of the read channel signal in response to an error in the read channel signal. Such a circuit would provide a more resiliant disk drive system by correcting for errors. In addition, integrated resistors consume expensive space on a device and a precise resistance value is difficult to achieve. Therefore, it would also be desirable that the circuit operate with external resistors. External resistors provide more precise resistance values at a reduced cost.